Wednesday, June 18, 2014

New paper finds another excuse for the 'pause': Cold nights getting colder in N Hemisphere

A paper published today in Environmental Research Letters finds a new excuse [#13 by my count] for the 15+ year 'pause' or 'hiatus' of global warming:

"a coherent cooling pattern across the Northern Hemisphere mid-latitudes that has emerged in the recent 15 years and is not reproduced by the models. This regional inconsistency between models and observations might be a key to understanding the recent hiatus in global mean temperature warming."

Thus, although warm extremes are getting warmer, cold nights in the N Hemisphere are getting colder, not warmer as predicted by global warming theory. How could increased CO2 cause "a coherent cooling pattern across the Northern Hemisphere mid-latitudes" over the past 15+ years? Inquiring minds want to know; according to the authors,

Excerpts from the discussion and conclusion:We analyzed observed and model-simulated trends inannual temperature extremes for the past 40 years(1971–2010) in comparison to the recent 15 years(1996–2010) using climate extreme indices from the HadEX2observational dataset and a large set of CMIP5 models.Simulated trends over the two periods are generally comparableto observed trends for absolute temperature extremes(i.e., coldest night (TNn) and warmest day (TXx) of the year)on a global scale. The observed trends in hot extremes (i.e.,TXx) are well represented in climate simulations, showingwarming trends similar to those seen in the observations inboth periods. Observed warming trends in cold extremes(TNn) are less well represented in climate simulations, butsimulated trends are nevertheless consistent with observedtrends globally and in many regions. The largest discrepancybetween observed and simulated trends in cold extremes isfound in the Northern mid-latitudes (20 °N–45 °N), whereobservations indicate a coherent zonal band of decreasingtrends over the recent 15 years. This might be connected tothe recent hiatus in the warming of global Tmean, which hasbeen characterized mainly as a winter phenomenon (e.g.,Kosaka and Xie 2013, Cohen et al 2012). Only a few individualmodel realizations simulate a cooling trend in TNn.Our findings are consistent with the suggestion that therecent 15-year period largely represents a highly unusual(extreme) realization of climate as part of internal variability(e.g., Meehl et al 2013, Kosaka and Xie 2013, Englandet al 2014). Other recent studies (Fyfe et al 2013, Fyfe andGillett 2014) argue that internal climate variability is unlikelyto be the only explanation for the discrepancy seen betweenmodel and observed trends and that some external forcingcomponents not fully represented in current climate modelscould have contributed to the local cooling trends in coldextremes. While precisely identifying the mechanisms behindthe observed regional cooling patterns in extreme cold temperatureslies beyond the scope of this paper, the resultspresented here provide relevant details to complete the overallpicture of recent temperature changes beyond globally averagedTmean.We conclude that while there appears to be a discrepancyin global Tmean trends between observations and simulationsover the hiatus period (e.g., Fyfe et al 2013), that discrepancydoes not generally extend to temperature extremes, with theexception of a recent cooling in Northern mid-latitude TNnthat is particularly apparent regionally in South Asia. Ingeneral, temperature extremes continue to increase in mostregions of the world consistent with the long-term projectionsunder global warming scenarios (e.g., Seneviratne et al 2012,Sillmann et al 2013b).

The discrepancy between recent observed and simulated trends in global mean surface temperature has provoked a debate about possible causes and implications for future climate change projections. However, little has been said in this discussion about observed and simulated trends in global temperature extremes. Here we assess trend patterns in temperature extremes and evaluate the consistency between observed and simulated temperature extremes over the past four decades (1971–2010) in comparison to the recent 15 years (1996–2010). We consider the coldest night and warmest day in a year in the observational dataset HadEX2 and in the current generation of global climate models (CMIP5). In general, the observed trends fall within the simulated range of trends, with better consistency for the longer period. Spatial trend patterns differ for the warm and cold extremes, with the warm extremes showing continuous positive trends across the globe and the cold extremes exhibiting a coherent cooling pattern across the Northern Hemisphere mid-latitudes that has emerged in the recent 15 years and is not reproduced by the models. This regional inconsistency between models and observations might be a key to understanding the recent hiatus in global mean temperature warming.